The present invention relates generally to the field of fluid drop generation and the application thereof to jet drop recorders of the type shown in U.S. Pat. No. 3,701,998 to Mathis, issued Oct. 31, 1972. In recorders of this type, one or more rows of orifices in a plate receive an electrically conductive recording fluid, such as a water base ink, from a pressurized fluid manifold and eject the fluid in parallel streams. The streams are broken up into drops as a result of the application of a series of travelling waves to the plate or other mechanical stimulation, such as the application of compression waves to the fluid in the manifold. Graphic reproduction in recorders of this type is accomplished by selectively charging and deflecting some of the drops in each of the streams and thereafter depositing drops on a moving web of paper or other material.
Charging of the drops is accomplished by application of charge control signals to charging electrodes near the edge of the drop streams. Charges are induced in the ends of fluid filaments emerging from the orifices by the charge control signals. As the drops separate from their parent fluid filaments, they carry a portion of the charge applied by the charging electrodes. Thereafter, the drops pass through electrostatic fields which have no effect on the uncharged drops but which cause the charged drops to be deflected. Drops which are not to be printed are charged sufficiently such that they are deflected to a catcher which services the drop streams.
One problem with ink jet printers has been attaining sufficient image resolution. Since a discrete number of drops are applied to form the images, it is clear that image definition may be improved by increasing the number of drops and providing a proportionate increase in data handling capability. If, however, only one print position on the print web is serviced by each orifice, the number of print positions per unit width and therefore the resolution of the image in the direction transverse to the web is limited by the minimum dimensions required between adjacent orifices. The approach taken in the Mathis device is to provide two rows of drop streams, with each row of streams being perpendicular to the direction of web movement and the drops from one row of the drop streams servicing print positions which interlace with those serviced by the other row of drop streams. The charging of drops in the two rows is timed such that printing from the two rows of streams is in registration. The separation between adjacent streams in each of the rows is therefore twice that which would be required in a one row printer of comparable resolution.
Another approach to improved resolution is shown in U.S. Pat. No. 3,739,395, to King, issued June 12, 1973, and assigned to the assignee of the present invention. In the King device, uncharged drops are caught and thus do not print, while charged drops from each orifice are deflected by two sets of deflection electrodes to a plurality of discrete positions on the moving web. Deflection of the drops is either perpendicular or parallel to the direction of web movement, or both, covering either a one-line matrix or a multiple-line matrix on the web. Since a number of print positions on the web are serviced by each jet, the distance between orifices may be greater than if each orifice serviced only one print position, while maintaining comparable resolution. The minimum required spacing between adjacent orifices is somewhat greater with the King device than with other prior art printers, however, since deflection electrodes must be positioned on all sides of each orifice.
U.S. Pat. No. 3,298,030, issued Jan. 10, 1967, to Lewis et al, discloses a multiple jet printer in which each jet services a number of print positions on the print web. The structure for generating deflection fields in the Lewis et al device, however, requires positioning electrodes between each adjacent jet, thereby limiting the minimum interjet spacing.
Another approach to increasing the effective resolution in an ink jet printer is shown in U.S. Pat. No. 4,010,477, issued Mar. 1, 1977, to Frey. In the Frey patent, a plurality of parallel rows of jets are provided, each row being positioned along a line which is inclined to the direction of web movement. Each row of jets services a group of print positions defining a band of adjacent print lines which extend along the length of the print web. The Frey printer is a binary printer; that is, each jet services only a single print position and the drops in each jet are charged to one of two discrete charge levels.
U.S. Pat. No. 4,085,409, issued April 18, 1978, to Paranjpe, discloses a printer which is somewhat similar in construction to that of the Mathis printer described above. The rows of jets in the Paranjpe printer are positioned along parallel lines which are inclined to the direction of web movement. Drops in each of the jet drop streams are selectively charged to several charge levels such that they are deflected to service a number of print positions. The inclined printer of Paranjpe provides improved resolution across the width of the web, both as a result of positioning the rows of jets along lines which are inclined with respect to the movement of the print web and by virtue of the fact that each jet services a number of print positions. Because of the deflection electrode position in the Paranjpe printer, the deflection fields are created by electrodes which do not extend between adjacent jets. The inter-jet spacing is therefore not limited by the deflection electrode structure.
In a printer such as shown in the Paranjpe patent, the drops from the drop streams may be accurately deposited at print positions on the web and thus provide a high resolution print image across the width of the web, providing that each of the jets is accurately positioned along the line of jets. Should one of the jets in such a system be slightly crooked, the band of print lines which it services will be laterally displaced from the desired position with the result that a small gap between the band serviced by the crooked jet and one of the adjacent bands of print lines will be produced in which no printing may be accomplished. Since most of the print image created by the printer will be of extremely high resolution, even small interband gaps will be noticeable and will detract significantly from the appearance of the final print image.
U.S. Pat. No. 4,060,804, issued Nov. 29, 1977, to Yamada, discloses an ink jet printing device in which two jets are provided for printing at two adjacent groups of print positions. In order to minimize the possibility of a gap or deterioration along the boundary of the bands of print lines serviced by the two jets, the jets are positioned such that the print lines in each of the groups adjacent the boundary are serviced by drops from the respective jets which are deflected the least. This arrangement is said to minimize the effect of errors in drop positioning which result from errors in the deflection of the drops. If one or both of the jets are crooked, however, this scheme will not provide an improvement in image quality.
It is seen, therefore, that a need exists for a printer in which each jet services a number of print positions forming a band of print lines along the print web and in which the effects of a crooked jet on the resulting print image are minimized.